They can be eaten either way. Some people use them as tea sandwiches, filling them with tuna or chicken salad, or something similar. I've seen recipes that fill them with caramelized onions, tapenade, even pulled pork barbecue.

You can add dough or finely chopped herbs, prosciutto, or mushrooms to the dough to introduce other flavors.

Cook's Illustrated has specific flavor combinations for them, which I cannot access because I'm not an online member.

And for those who would like to try making them, allow me to suggest using this recipe for the purposes of learning how to make the dough. The instructions are much easier to understand than other sites. My thanks to Alton Brown, who taught me how to make the dough -